1. Field of the Invention
The present invention relates to a device for reading and writing data on magnetic recording layer provided on photo film, and more particularly to a magnetic read-write device suitable for incorporation into a camera.
2. Description of the Related Art
A new type photo film has been developed with a magnetic recording layer to permit recording information about each picture frame such as shutter speed and aperture size, the number of prints to make, and trimming range. A magnetic head incorporated in a camera is utilized for the recording on the photo film while it is advanced one frame after exposure, as disclosed for example in JPA 7-219021. The information recorded on the magnetic recording layer of the photo film is read by a magnetic head installed in a photo printer, for use in controlling the exposure therein.
Since the new type photo film is contained in a cartridge shell that allows to advance the leader of the photo film to the outside after winding up the entire length of the photo film, it is possible to wind up the photo film that is not completely exposed back into the cartridge shell and remove the cartridge from the camera to load it in another camera, or to replace it by another cartridge. When to use the cartridge containing the half-exposed photo film, it is necessary to search for an unexposed portion that follows to the still exposed frame, and place it behind an exposure aperture. By tracing the magnetic tracks on the magnetic recording layer that are formed along the exposed frames, it is possible to discriminate between exposed and unexposed portions of the photo film. Therefore, it is desirable to provide the camera for the new type photo film with a magnetic reading function. The magnetic reading function in the camera is desirable because it also permits checking the recording condition on the photo film immediately after the recording.
Because the photo filmstrips, especially the new type ones, are stiffer, and the magnetic recording layer on the photo film has lower magnetic density in comparison with ordinary magnetic recording tapes, it is desirable for stable and reliable reading to use a magnetic reading head with a large turn number. However, providing a reading head in addition to a recording head results in raising the cost, weight and size of the camera.
Using a magnetic head for both reading and writing simply by increasing its turn number is possible, but the reliability of that magnetic head cannot be sufficient because the current through the coil of the head decreases with an increase of the direct current resistance that increases with the turn number, so that the current is more influenced by the noises. To use thicker wire to lower the direct current resistance of the coil enlarges the magnetic head.
A read-write magnetic head with a reading coil and a writing coil on a core is known, wherein the reading coil has a larger turn number, and the coils are used alternatively so that the signal is reproduced at sufficiently high level, while the current for writing is maintained in a proper level. There are four-terminal type read-write head and three-terminal type read write-head. In the four-terminal type, the reading coil is separate from the writing coil, both having respective terminals. In the three-terminal type, the reading coil is included in the writing coil.
In either case, the turn number of the reading coil is usually more than ten times that of the writing coil. Therefore, when the writing coil is driven with a voltage of about 3 volt, for instance, voltage of several tens volt will be induced between the terminals of the reading coils. Such an induced voltage or current would break down operation amplifiers that are connected to the reading coil to amplify the reproduced signal. To protect the operation amplifiers from the induced voltage, a protection circuit is connected between an inverting input terminal and a non-inverting input terminal of a preamplifier. The protection circuit has a pair of Zener diodes oriented reversely and connected in series or in parallel to each other. Otherwise, it is necessary to provide a selector switch to disconnect the coil of the magnetic head from the amplification circuit during the writing, and from a head drive circuit during the reading.
Connecting the protection circuit or the selector switch in the input of the amplification circuit that amplifies the reproduced signal can put a certain noise on the reproduced signal. Moreover, the Zener diodes or the selector switch and the magnetic head having the two kinds of coils on the single core itself are relatively expensive.